With advances in ball grid array (BGA) technology, there is an increasing pressure to mount denser and higher performing devices on a circuit board. One popular BGA package or device is the ceramic BGA (CBGA) package, where an integrated circuit (IC) is mounted on a ceramic circuit board to take advantage of the electrical and thermal benefits offered by ceramic substrates over conventional plastic substrates.
One feature of constructing a CBGA package that is typically critical is the precise alignment of small solder balls that serve as interconnects between the package and a circuit board on which it is mounted. That is, the shape, position, and dimensional tolerances of both the solder balls, and the solder pads formed on the package to which the solder balls are attached, are often critical in producing a precisely defined and aligned ball grid array on the package.
The majority of CBGA packages are formed utilizing a multi-layer co-fired ceramic (MLCC) technology, where solder pads are deposited on a ceramic substrate in its “green” (soft) state. Subsequently, the ceramic substrate is fired and hardened. However, during firing, there is limited control over the shrinkage of the ceramic substrate, and the definition and alignment of the solder pads on the hardened ceramic may not match that of the pattern of solder pads that was originally applied to the “green” ceramic. See, for example, the before and after solder pad patterns shown in FIGS. 1 and 2, where the pattern shown in FIG. 2 illustrates the skew of solder pads (e.g., pads 102, 104, 106) as a result of shrinkage of the ceramic substrate 100.
A second process for depositing solder pads on a substrate is to screen print them on a hardened substrate (whether ceramic or otherwise). Thereafter, only the solder pads are affected by a curing or firing process. However, the relative positional accuracy and dimensional control of the solder pads are limited by factors such as screen stretch (leading to positional drifts) and screen mesh interference (leading to imprecise solder pad shapes, or slumping of a thick film after deposition). See, for example, the solder pad pattern shown in FIG. 3, where the pattern of solder pads shown in FIG. 1 has not been achieved on the substrate 300 as a result of screen stretch during the deposition of the solder pads (e.g., pads 302, 304, 306).